(i) Field of the Invention
The present invention relates to coating systems for the generation of protective surface alloys for high temperature metal alloy products and, more particularly, relates to the provision of overlay metal alloy coatings on the internal wall surfaces of high-temperature stainless steel tubes to produce a coating that provides corrosion resistance and reduces the formation of catalytic coking in hydrocarbon processing such as in olefin production and in direct reduction of ores.
(ii) Description of the Related Art
Stainless steels are a group of alloys based on iron, nickel and chromium as the major constituents, with additives that can include carbon, tungsten, niobium, titanium, molybdenum, manganese, and silicon to achieve specific structures and properties. The major types are known as martensitic, ferritic, duplex and austenitic steels. Austenitic stainless steel generally is used where both high strength and high corrosion resistance is required. One group of such steels is known collectively as high temperature alloys (HTAs) and is used in industrial processes that operate at elevated temperatures generally above 650xc2x0 C. and extending to the temperature limits of ferrous metallurgy at about 1150xc2x0 C. The major austenitic alloys used have a composition of iron, nickel or chromium in the range of 18 to 42 wt. % chromium, 18 to 48 wt. % nickel, balance iron and other alloying additives. Typically, high chromium stainless steels have about 31 to 38 wt % chromium and low chromium stainless steels have about 20 to 25 wt % chromium.
The bulk composition of HTAs is engineered towards physical properties such as creep resistance and strength, and chemical properties of the surface such as corrosion resistance. Corrosion takes many forms depending on the operating environment and includes carburization, oxidation and sulfidation. Protection of the bulk alloy is often provided by the surface being enriched in chromium oxide (chromia). The specific compositions of the alloys used represent an optimization of physical properties (bulk) and chemical properties (surface). The ability of addressing the chemical properties of the surface through a surface alloy, and physical properties through the bulk composition, would provide great opportunities for improving materials performance in many severe service industrial environments.
Surface alloying can be carried out using a variety of coating processes to deliver the right combination of materials to the component""s surface at an appropriate rate. These materials would need to be alloyed with the bulk matrix in a controlled manner that results in a microstructure capable of providing the pre-engineered or desired benefits. This would require control of the relative interdiffusion of all constituents and the overall phase evolution. Once formed, the surface alloy can be activated and reactivated, as required, by a reactive gas thermal treatment. Since both the surface alloying and the surface activation require considerable mobility of atomic constituents at temperatures greater than 700xc2x0 C., HTA products can benefit most from the procedure due to their designed ability of operating at elevated temperatures. The procedure can also be used on products designed for lower operating temperatures, but may require a post heat treatment after surface alloying and activation to reestablish physical properties.
Surface alloys or coating systems can be engineered to provide a full range of benefits to the end user, starting with a commercial base alloy chemical composition and tailoring the coating system to meet specific. performance requirements. Some of the properties that can be engineered into such systems include: superior hot gas corrosion resistance (carburization, oxidation, sulfidation); controlled catalytic activity; and hot erosion resistance.
Two metal oxides are mainly used to protect alloys at high temperatures, namely chromia and alumina, or a mixture of the two. The compositions of stainless steels for high temperature use are tailored to provide a balance between good mechanical properties and good resistance to oxidation and corrosion. Alloy compositions which can provide an alumina scale are favoured when good oxidation resistance is required, whereas compositions capable of forming a chromia scale are selected for resistance to hot corrosive conditions. Unfortunately, the addition of high levels of aluminum and chromium to the bulk alloy is not compatible with retaining good mechanical properties and coatings containing aluminum and/or chromium normally are applied onto the bulk alloy to provide the desired surface oxide.
One of the most severe industrial processes from a materials perspective is the manufacture of olefins such as ethylene by hydrocarbon steam pyrolysis (cracking). Hydrocarbon feedstock such as ethane, propane, butane or naphtha is mixed with steam and passed through a furnace coil made from welded tubes and fittings. The coil is heated on the outerwall and the heat is conducted to the innerwall surface leading to the pyrolysis of the hydrocarbon feed to produce the desired product mix at temperatures in the range of 850 to 1150xc2x0 C. An undesirable side effect of the process is the buildup of coke (carbon) on the innerwall surface of the coil. There are two major types of coke: catalytic coke (or filamentous coke) that grows in long threads when promoted by a catalyst such as nickel or iron, and amorphous coke that forms in the gas phase and plates out from the gas stream. In light feedstock cracking, catalytic coke can account for 80 to 90% of the deposit and provides a large surface area for collecting amorphous coke.
The coke can act as a thermal insulator, requiring a continuous increase in the tube outerwall temperature to maintain throughput. A point is reached when the coke buildup is so severe that the tube skin temperature cannot be raised any further and the furnace coil is taken offline to remove the coke by burning it off (decoking). The decoking operation typically lasts for 24 to 96 hours and is necessary once every 10 to 90 days for light feedstock furnaces and considerably longer for heavy feedstock operations. During a decoke period, there is no marketable production which represents a major economic loss. Additionally, the decoke process degrades tubes at an accelerated rate, leading to a shortened lifetime. In addition to inefficiencies introduced to the operation, the formation of coke also leads to accelerated carburization, other forms of corrosion, and erosion of the tube innerwall. The carburization results from the diffusion of carbon into the steel forming brittle carbide phases. This process leads to volume expansion and the embrittlement results in loss of strength and possible crack initiation. With increasing carburization, the alloy""s ability of providing some coking resistance through the formation of a chromium based scale deteriorates. At normal operating temperatures, half of the wall thickness of some steel tube alloys can be carburized in as little as two years of service. Typical tube lifetimes range from 3 to 6 years.
It has been demonstrated that aluminized steels, silica coated steels, and steel surfaces enriched in manganese oxides or chromium oxides are beneficial in reducing catalytic coke formation ALONIZING(trademark), or aluminizing, involves the diffusion of aluminum into the alloy surface by pack cementation, a chemical vapour deposition technique. The coating is functional to form a NiAl type compound and provides an alumina scale which is effective in reducing catalytic coke formation and protecting from oxidation and other forms of corrosion. The coating is not stable at temperatures such as those used in ethylene furnaces, and also is brittle, exhibiting a tendency to spall or diffuse into the base alloy matrix. Generally, pack cementation is limited to the deposition of one or two elements, the co-deposition of multiple elements being extremely difficult. Commercially, it is generally limited to the deposition of only a few elements, mainly aluminum. Some work has been carried out on the codeposition of two elements, for example chromium and silicon. Another approach to the application of aluminum diffusion coatings to an alloy substrate is disclosed in U.S. Pat. No. 5,403,629 issued to P. Adam et al. This patent details a process for the vapour deposition of a metallic interlayer on the surface of a metal component, for example by sputtering. An aluminum diffusion coating is thereafter deposited on the interlayer.
Alternative diffusion coatings have also been explored. In an article in xe2x80x9cProcessing and Propertiesxe2x80x9d entitled xe2x80x9cThe Effect of Time at Temperature on Silicon-Titanium Diffusion Coating on IN738 Base Alloyxe2x80x9d by M. C. Meelu and M. H. Lorretto, there is disclosed the evaluation of a Sixe2x80x94Ti coating, which had been applied by pack cementation at high temperatures over prolonged time periods.
The benefits of aluminising an MCrAlX coating on superalloys for improved oxidation and corrosion resistance have been previously well documented. European Patent EP 897996, for example, describes the improvement of high temperature oxidation resistance of an MCrAlY on a superalloy by the application of an aluminide top coat using chemical vapour deposition techniques. Similarly, U.S. Pat. No. 3,874,901 describes a coating system for superalloys including the deposition of an aluminum overlay onto an MCrAlY using electron beam-physical vapour deposition to improve the hot corrosion and oxidation resistance of the coating by both enriching the near-surface ofthe MCrAlY with aluminum and by sealing structural defects in the overlay. Both of these systems relate to improvement of oxidation and/or hot corrosion resistance imparted to superalloys by the deposition of an MCrAlY thereon. These references do not relate to improvement of anticoking properties or corrosion resistance of high temperature stainless steel alloys used in the petrochemical industries.
A major difficulty in seeking an effective coating is the propensity of many applied coatings to fail to adhere to the tube alloy substrate under the specified high temperature operating conditions in hydrocarbon pyrolysis furnaces. Additionally, the coatings lack the necessary resistance to any or all of thermal stability, thermal shock, hot erosion, carburization, oxidation and sulfidation. A commercially viable product for olefins manufacture by hydrocarbon steam pyrolysis and for direct reduction of iron ores must be capable of providing the necessary coking and carburization resistance over an extended operating life while exhibiting thermal stability, hot erosion resistance and thermal shock resistance.
It is therefore a principal object of the present invention to impart beneficial properties to HTAs through surface alloying to substantially eliminate or reduce the catalytic formation of coke on the internal surfaces of tubing, piping, fittings and other ancillary furnace hardware by minimizing the number of sites for catalytic coke formation and by improving the quality of alumina scale on surface alloy coatings deposited on high temperature stainless steels. The alloy coatings of the invention are particularly suited for the manufacture of olefins by hydrocarbon steam pyrolysis, typified by use in furnace tubes and fittings, for ethylene production, the manufacture of other hydrocarbon-based products in the petrochemical industries, and in the direct reduction of ores such as typified by the direct reduction of iron oxide ores to metallic iron in carbon-containing atmospheres.
It is another object of the invention to increase the carburization resistance of HTAs used for tubing, piping, fittings and ancillary furnace hardware whilst in service.
It is a further object of the invention to augment the longevity of the improved performance benefits derived from the surface alloying under commercial conditions by providing thermal stability, hot erosion resistance and thermal shock resistance.
In accordance with the present invention there are provided three embodiments of surface alloy structures generatable from the deposition of a MCrAlX alloy overlay directly on a high temperature stainless steel alloy substrate or onto an intermediary diffusion coating, followed by heat treatment to establish the coating microstructure and to metallurgically bond the coatings to the substrate.
The first embodiment of surface alloy structure of the invention comprises the application of a MCrAlX (where M=nickel, cobalt, iron or a mixture thereof and X=yttrium, hafnium, zirconium, lanthanum or combination thereof) overlay material onto a high temperature stainless steel alloy substrate and an appropriate heat treatment of the MCrAlX overlay and the substrate.
The second embodiment of surface alloy structure of the invention comprises depositing a layer of aluminum on the said MCrAlX overlay and heat treating the composite of aluminum, MCrAlX overlay and substrate to establish the coating microstructure.
The third embodiment of surface alloy structure of the invention comprises depositing a diffusion coating onto the high temperature stainless steel alloy substrate beneath the MCrAlX overlay. The nitrogen and carbon contents of standard high temperature stainless steel alloys can lead to the formation of undesirable brittle nitride and carbide layers at the coating/substrate interface. The deposition of a diffusion coating, containing a stable nitride former, beneath the MCrAlX coating will act to disperse nitride precipitates. This is more desirable than a continuous nitride layer. The diffusion coating will also act to disperse carbide precipitates. Again this is more desirable than a continuous carbide layer at the coating/substrate interface. The diffusion coating will also increase the adherence of the MCrAlX to the substrate and decreases the level surface preparation necessary for coating deposition. The MCrAlX overlay alloy is deposited onto the diffusion coating, an aluminum layer is deposited onto the MCrAlX overlay, and the coating system is heat-treated to diffuse aluminum into the overlay and to metallurgically bond the layers together and to the substrate and to achieve a desired metallurgical microstructure.
Each of the above embodiments optionally is pre-oxidized to form a protective outer layer of predominantly xe2x88x9d-alumina. The xe2x88x9d-alumina layer is highly effective at reducing or eliminating catalytic coke formation. These surface alloys are compatible with high temperature commercial processes at temperatures of up to 1150xc2x0 C. such as encountered in olefin manufacturing by hydrocarbon steam pyrolysis typified by ethylene production.
In its broad aspect, the method of the invention for providing a protective and inert coating to high temperature stainless steels at temperatures up to 1150xc2x0 C. comprises depositing onto a high temperature stainless steel substrate a continuous overlay coating of a MCrAlX alloy, where M=nickel, cobalt or iron or mixture thereof and X=yttrium, hafnium, zirconium, lanthanum or combination thereof, having about 10 to 25 wt % chromium, about 8 to 15 wt % aluminum and up to about 3.0 wt %, preferably about 0.25 to 1.5 wt % of yttrium, hafnium, zirconium, lanthanum or combination thereof, the balance M. The overlay coating may be deposited by a variety of methods including but not limited to thermal spray, physical vapour deposition and slurry coating techniques. The overlay coating and substrate are heat-treated at a soak temperature in the range of about 1000 to 1160xc2x0 C. for about 20 minutes to 24 hours.
The overlay coating is deposited in a thickness of about 50 to 350 xcexcm, preferably in a thickness about 120 to 250 xcexcm, and most preferably about 150 xcexcm, such as by magnetron sputtering physical vapour deposition or thermal spray onto the substrate at a temperature in the range of about 200 to 1000xc2x0 C., preferably at about 450xc2x0 C., and the overlay coating and substrate heated to a desired soak temperature. Preferably, the MCrAlX is NiCrAlY and has, by weight, about 12 to 22% chromium, about 8 to 13% aluminum and about 0.8 to 1% yttrium, the balance nickel.
The high temperature stainless steel substrate comprises, by weight, 18 to 38% chromium, 18 to 48% nickel, the balance iron and alloying additives, and preferably is a high chromium stainless steel having 31 to 38 wt % chromium or a low chromium steel having 20 to 25 wt % chromium.
In accordance with a further embodiment of the method of the invention, a continuous diffusion coating is deposited beneath the overlay to minimize or avoid the formation of continuous nitride or carbide layers at the coating/substrate interface. An effective diffusion coating is comprised of about 35 to 45 wt % aluminum, a total of about 5 to 20 wt % of at least one of titanium or chromium, and 40 to 55 wt % silicon, prefeably about 35 to 40 wt 5 aluminum, about 5 to 15 wt % titanium and about 50 to 55 wt % silicon, is deposited onto a high temperature stainless steel substrate as described in U.S. Pat. No. 6,093,260 issued Jul. 25, 2000 incorporated herein by reference, a continuous MCrAlX overlay alloy coating is deposited onto the diffusion coating, and an aluminum layer is deposited onto the overlay alloy coating.
In accordance with a further embodiment of the method of the invention, a continuous diffusion coating is deposited beneath the overlay to minimize or avoid the formation of continuous nitride or carbide layers at the coating/substrate interface. An effective diffusion coating is comprised of about 35 to 45 wt % aluminum, a total of about 5 to 20 wt % of at least one of titanium or chromium, and 40 to 55 wt % silicon, preferably about 35 to 40 wt % aluminum, about 5 to 15 wt % titanium and about 50 to 55 wt % silicon, is deposited onto a high temperature stainless steel substrate as described in application Ser. No. 08/839,831 now U.S. Pat. No. 6,093,260 incorporated herein by reference, a continuous MCrAlX overlay alloy coating is deposited onto the diffusion coating, and an aluminum layer is deposited onto the overlay alloy coating.
The diffusion coating preferably is deposited by physical vapour deposition at a temperature in the range of 400 to 600xc2x0 C. or 800 to 900xc2x0 C., preferably at either 450 or 850xc2x0 C. Thermal spray deposition techniques also may be used. The diffusion coating is then heated to a soak temperature at a rate of temperature rise of at least 5 Celsius degrees/minute, preferably at a rate of 10 to 20 Celsius degrees/minute, to establish the coating microstructure. The overlay coating, and preferably an aluminum layer, are deposited such as by physical vapour deposition onto the diffusion coating and then heat-treated to establish the multiphased microstructure and to metallurgically bond the coatings.
The systems subsequently can be heated in an oxygen-containing atmosphere at a temperature above about 1000xc2x0 C., preferably in the range of above 1000xc2x0 C. to 1160xc2x0 C., in a consecutive step or in a separate later step for a time effective to form a surface layer of xe2x88x9d-alumina thereon.
The diffusion coating is deposited in a thickness of about 20 to 100 xcexcm and preferably in a thickness of about 20 to 60 xcexcm. The diffusion coating is heat-treated at a soak temperature in the range of about 1030 to 1150xc2x0 C. for a time effective to form an enrichment pool containing about 3 to 7 wt % silicon and about 5 to 15 wt % aluminum with the balance thereof being chromium, titanium, iron, nickel and any base alloy additives and a diffusion barrier between the base alloy and enrichment pool containing intermetallics of silicon and one or more of titanium or aluminum and the base alloying elements. Preferably, the diffusion barrier contains about 6 to 10 wt % silicon, 0 to 5 wt % aluminum, 0 to 4 wt % titanium and about 25 to 50 wt % chromium, the balance iron and nickel and any base alloying elements.
An alternative process for creating a similar coating system is the deposition of the diffusion coating, overlay, and optionally an aluminum layer in sequence, and heat-treating in an inert atmosphere at a soak temperature in the range of about 1030 to 1160xc2x0 C. to establish the microstructure and to bond the coatings.